Using 2 Source Units for Audio

Hi all I'd like to install a portable dvd player that I have into my car so I can dvd mp3 discs. I don't have any monitors in the car but I just want it to play mp3's anyways no video. I have a standard setup with aftermarket head unit and speakers. I'm just not sure how I could go about installing it without a head unit that has a screen with inputs/outputs and a source selector. I read this guy installed a ps2 with a fm modulator for sound (I'm a bit iffy on the quality though running through an fm signal)

Anybody have any ideas.
Thanks for the help in advance

what HU do you have? a lot of aftermarket units have either auxillary inputs, or changer controllers that can be retrofitted to auxillary inputs. If your unit does not have something along those lines, you can use an external equalizer with multiple-input capability to control output to the amps, or use an FM modulator. A decent modulator shouldn't sound too bad, you'd be surprised what kind of quality you can get from one.